Q. Does this allow you to identify the mill at where a document was produced?

A. Not exactly. This technique allows you to identify an individual document that has been seen before. Each individual sheet of paper is unique, due in part to its unique surface texture, that is, the detailed three-dimensional shape of its surface.

Q. How can you measure the 3-D structure of a sheet of paper? I thought paper was flat.

A. Paper looks flat to the naked eye, but viewed through a microscope it looks like a tangled mat of fibers, with a complex shape. The shape of the surface (the “surface texture”) depends on the exact layout of fibers, which is unique to that individual piece of paper.

Q. Does this method require modification of a scanner?

A. No. You can measure a document’s surface texture using an ordinary, unmodified, desktop scanner. We used a midrange scanner, an Epson v700, in most of our experiments.

Q. What types of paper does this work on?

A. We have shown that this technique works on a variety of paper types. Our study shows that this technique works on a variety of different types of paper including university quality letterhead with watermark, index cards, and normal copy paper.